3.1. Pathobiology of MDS
Myelodysplastic syndrome (“MDS”) refers to a diverse group of hematopoietic stem cell disorders. MDS is characterized by a cellular marrow with impaired morphology and maturation (dysmyelopoiesis), peripheral blood cytopenias, and a variable risk of progression to acute leukemia, resulting from ineffective blood cell production. The Merck Manual 953 (17th ed. 1999) and List et al., 1990, J. Clin. Oncol. 8:1424.
The initial hematopoietic stem cell injury can be from causes such as, but not limited to, cytotoxic chemotherapy, radiation, virus, chemical exposure, and genetic predisposition. A clonal mutation predominates over bone marrow, suppressing healthy stem cells. In the early stages of MDS, the main cause of cytopenias is increased programmed cell death (apoptosis). As the disease progresses and converts into leukemia, gene mutation rarely occurs and a proliferation of leukemic cells overwhelms the healthy marrow. The disease course differs, with some cases behaving as an indolent disease and others behaving aggressively with a very short clinical course that converts into an acute form of leukemia.
About 15,000 people are diagnosed with MDS each year in the United States, and about 60,000 are living with the disease. MDS was first considered a distinct disease in 1976, and occurrence was estimated at 1500 new cases every year. At that time, only patients with less than five percent blasts were considered to have this disorder. Statistics from 1999 estimated 13,000 new cases per year and about 1000 cases per year in children, surpassing chronic lymphocytic leukemia as the most common form of leukemia in the western hemisphere. The perception that the incidence is increasing may be due to improvements in recognition and criteria for diagnosis. The disease is found worldwide.
An international group of hematologists, the French-American-British (FAB) Cooperative Group, classified MDS disorders into five subgroups, differentiating them from acute myeloid leukemia. The Merck Manual 954 (17th ed. 1999); Bennett J. M., et al., Ann. Intern. Med. 1985 October, 103(4): 620-5; and Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617. An underlying trilineage dysplastic change in the bone marrow cells of the patients is found in all subtypes.
There are two subgroups of refractory anemia characterized by five percent or less myeloblasts in bone marrow: (1) refractory anemia (RA) and; (2) RA with ringed sideroblasts (RARS), defined morphologically as having 15% erythroid cells with abnormal ringed sideroblasts, reflecting an abnormal iron accumulation in the mitochondria. Both have a prolonged clinical course and low incidence of progression to acute leukemia. Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617.
There are two subgroups of refractory anemias with greater than five percent myeloblasts: (1) RA with excess blasts (RAEB), defined as 6-20% myeloblasts, and (2) RAEB in transformation (RAEB-T), with 21-30% myeloblasts. The higher the percentage of myeloblasts, the shorter the clinical course and the closer the disease is to acute myelogenous leukemia. Patient transition from early to more advanced stages indicates that these subtypes are merely stages of disease rather than distinct entities. Elderly patients with MDS with trilineage dysplasia and greater than 30% myeloblasts who progress to acute leukemia are often considered to have a poor prognosis because their response rate to chemotherapy is lower than de novo acute myeloid leukemia patients. The World Health Organization (WHO) classification (1999) proposes to include all cases of RAEB-T, or patients with greater than 20% myeloblasts, in the category of acute leukemia because these patients have similar prognostic outcomes. However, their response to therapy is worse than the de novo or more typical acute myelogenous leukemia or acute nonlymphocytic leukemia (ANLL) patient. Id.
The fifth type of MDS, the most difficult to classify, is called chronic myelomonocytic leukemia (CMML). This subtype can have any percentage of myeloblasts but presents with a monocytosis of 1000/dL or more. It may be associated with splenomegaly. This subtype overlaps with a myeloproliferative disorder and may have an intermediate clinical course. It is differentiated from the classic chronic myelocytic leukemia (CML) that is characterized by a negative Ph chromosome. The recent WHO classification (1999) proposes that juvenile and proliferative CMML be listed separately from FAB under MDS/myeloproliferative disorder (MPD) with splenomegaly and greater than 13,000 total WBC. CMML is limited to monocytosis, less than 13,000/mm3 total leukocytes, and requires trilineage dysplasia. Id. Harris N. L., et al., J. Clin. Oncol. 1999 December, 17(12): 3835-49. Finally, some other international organizations, including WHO, have suggested a sixth class of MDS patients, characterized by a del (5q) abnormality.
MDS is primarily a disease of elderly people, with the median onset in the seventh decade of life. The median age of these patients is 65 years, with ages ranging from the early third decade of life to as old as 80 years or older. The syndrome may occur in any age group, including the pediatric population. Patients who survive malignancy treatment with alkylating agents, with or without radiotherapy, have a high incidence of developing MDS or secondary acute leukemia. About 60-70% of patients do not have an obvious exposure or cause for MDS, and are classified as primary MDS patients.
The most common cases of MDS are primary, or idiopathic. However, a nonspecific history of exposure to indeterminable chemicals or radiation 10-15 years prior to onset of disease may be present in about 50% of patients. This relationship to pathogenesis remains unproved. Compounds such as, but not limited to, benzene, insecticides, weed killers, and fungicides are possible causes of MDS. Goldberg H., et al., Cancer Res. 1990 Nov. 1; 50(21): 6876-81. Secondary MDS describes development of MDS or acute leukemia after known exposures to chemotherapy drugs that can cause bone marrow damage. These drugs are associated with a high incidence of chromosomal abnormalities following exposure and at the time of MDS or acute leukemia diagnosis.
Further, MDS is associated with complications associated with severe cytopenias. Other complications are development of myelofibrosis, which can accelerate decline in blood counts and increase transfusion requirements. Transformation to acute leukemia accelerates the development of complications such as anemia, bleeding, and infections.
The International MDS Risk Analysis (IMRA) Workshop proposed an International Prognosis Scoring System (IPSS) to decrease imprecision in predicting survival and AML risk in MDS patients. The IPSS is based on the number of cytopenias, percentage of BM blasts, and type of cytogenetic abnormalities (Table 1). Greenberg et al., Blood 1997, 89:2079-88. The latter are categorized into good (normal, −Y, del (5q), del (20q)), intermediate, and poor subgroups (complex or chromosome 7 abnormalities).
TABLE 1International Prognostic Scoring System for MDSScore ValuePrognostic00.51.01.52.0VariableBone marrow<55-10—11-2021-30blasts (%)Karyotype*GoodIntermediatePoorCytopenias0/12/3*Good, normal, del (5q), del (20q), -Y; Poor, complex (>3) or chromosome 7 abnormalities; Intermediate, +8, and other single or double abnormalities.
3.2. MDS Treatment
The current treatment of MDS is based on the stage and the mechanism of the disease that predominates the particular phase of the disease process. Bone marrow transplantation has been used in patients with poor prognosis or late-stage MDS. Epstein and Slease, 1985, Surg. Ann. 17:125. This type of therapy, however, is both painful for donor and recipient, because of the involvement of invasive procedures and can cause severe and even fatal complications to the recipient, particularly with allogeneic transplant and related Graft Versus Host Disease (GVHD) results. Therefore, the risk of GVHD restricts the use of bone marrow transplantation to patients with otherwise fatal diseases. Further, as most patients are elderly and only a few young MDS patients will have a matched donor, the use of bone marrow transplantation is limited.
An alternative approach to therapy for MDS is the use of hematopoietic growth factors or cytokines to stimulate blood cell development in a recipient. Dexter, 1987, J. Cell Sci. 88:1; Moore, 1991, Annu. Rev. Immunol. 9:159; and Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617. The process of blood cell formation, by which a small number of self-renewing stem cells give rise to lineage specific progenitor cells that subsequently undergo proliferation and differentiation to produce the mature circulating blood cells has been shown to be at least in part regulated by specific hormones. These hormones are collectively known as hematopoietic growth factors. Metcalf, 1985, Science 229:16; Dexter, 1987, J. Cell Sci. 88:1; Golde and Gasson, 1988, Scientific American, July: 62; Tabbara and Robinson, 1991, Anti-Cancer Res. 11:81; Ogawa, 1989, Environ. Health Presp. 80:199; and Dexter, 1989, Br. Med. Bull. 45:337. The most well characterized growth factors include erythropoietin (EPO), granulocyte macrophage colony stimulating factor (GM-CSF), and granulocyte colony stimulating factor (G-CSF). Apart from inducing proliferation and differentiation of hematopoietic progenitor cells, such cytokines have also been shown to activate a number of functions of mature blood cells, including influencing the migration of mature hematopoietic cells. Stanley et al., 1976, J. Exp. Med. 143:631; Schrader et al., 1981, Proc. Natl. Acad. Sci. U.S.A. 78:323; Moore et al., 1980, J. Immunol. 125:1302; Kurland et al., 1979, Proc. Natl. Acad. Sci. U.S.A. 76:2326; Handman and Burgess, 1979, J. Immunol. 122:1134; Vadas et al., 1983, Blood 61:1232; Vadas et al., 1983, J. Immunol. 130:795; and Weibart et al., 1986, J. Immunol. 137:3584.
Unfortunately, hematopoietic growth factors have not proven effective in many clinical settings. Clinical trials of MDS patients treated with recombinant human GM-CSF and G-CSF have shown that while these cytokines can restore granulocytopoiesis in treated patients, their efficacy is restricted to the granulocyte or monocyte lineage with little or no improvement in hemoglobin or platelet counts. Schuster et al., 1990, Blood 76 (Suppl. 1):318a. When such patients were treated with recombinant human EPO, a sustained improvement in hemoglobin or decrease in transfusion requirement was achieved in only less than 25% of patients. Besa et al., 1990, 76 (Suppl. 1):133a; Hellstrom et al., 1990, 76 (Suppl. 1):279a; Bowen et al., 1991, Br. J. Haematol. 77:419. Therefore, there remains a need for safe and effective methods of treating and managing MDS.
3.3. Thalidomide and Azacitidine
Thalidomide is a racemic compound sold under the tradename Thalomid® and chemically named α-(N-phthalimido)glutarimide or 2-(2,6-dioxo-3-piperidinyl)-1H-isoindole-1,3(2H)-dione. Thalidomide was originally developed in the 1950's to treat morning sickness, but due to its teratogenic effects was withdrawn from use. Thalidomide has been approved in the United States for the acute treatment of the cutaneous manifestations of erythema nodosum leprosum in leprosy. Physicians' Desk Reference, 1154-1158 (56th ed., 2002). Because its administration to pregnant women can cause birth defects, the sale of thalidomide is strictly controlled. Id. Thalidomide has reportedly been studied in the treatment of other diseases, such as chronic graft-vs-host disease, rheumatoid arthritis, sarcoidosis, several inflammatory skin diseases, and inflammatory bowel disease. See generally, Koch, H. P., Prog. Med. Chem. 22:165-242 (1985). See also, Moller, D. R., et al., J. Immunol. 159:5157-5161 (1997); Vasiliauskas, E. A., et al., Gastroenterology 117:1278-1287 (1999); Ehrenpreis, E. D., et al., Gastroenterology 117:1271-1277 (1999). It has further been alleged that thalidomide can be combined with other drugs to treat ischemia/repercussion associated with coronary and cerebral occlusion. See U.S. Pat. No. 5,643,915, which is incorporated herein by reference.
More recently, thalidomide was found to exert immunomodulatory and anti-inflammatory effects in a variety of disease states, cachexia in AIDS, and opportunic infections in AIDS. In studies to define the physiological targets of thalidomide, the drug was found to have a wide variety of biological activities exclusive of its sedative effect including neurotoxicity, teratogenicity, suppression of TNF-α production by monocytes/macrophages and the accompanying inflammatory toxicities associated with high levels of TNF-α, and inhibition of angiogenesis and neovascularization.
Additionally, beneficial effects have been observed in a variety of dermatological conditions, ulcerative colitis, Crohn's disease, Bechets's syndrome, systemic lupus erythematosis, aphthous ulcers, and lupus. The anti-angiogenic properties of thalidomide in in vivo models have been reported. D'Amato et al., Thalidomide Is An Inhibitor Of Angiogenesis, 1994, PNAS, USA 91:4082-4085.
One of the most therapeutically significant potential uses of thalidomide is in the treatment of cancer. The compound has been investigated in the treatment of various types of cancer, such as refractory multiple myeloma, brain, breast, colon, and prostate cancer, melanoma, mesothelioma, and renal cell carcinoma. See, e.g., Singhal, S., et al., New England J. Med. 341(21):1565-1571 (1999); and Marx, G. M., et al., Proc. Am. Soc. Clin. Oncology 18:454a (1999). Thalidomide reportedly can also be used to prevent the development of chronic cardiomyopathy in rats caused by doxorubicin. Costa, P. T., et al., Blood 92(10:suppl. 1):235b (1998). Other reports concerning the use of thalidomide in the treatment of specific cancers include its combination with carboplatin in the treatment of glioblastoma multiforme. McCann, J., Drug Topics 41-42 (Jun. 21, 1999). The use of thalidomide in combination with dexamethasone reportedly was effective in the treatment of patients suffering from multiple myeloma who also received, as supportive care, human granulocyte colony-stimulating factor (G-CSF), ciprofloxacin, and non-absorbable antifungal agents. Kropff, M. H., Blood 96(11 part 1):168a (2000); see also, Munshi, N. et al., Blood 94(10 part 1):578a (1999). Other chemotherapy combinations that comprise thalidomide are disclosed in International Application No. PCT/US01/15326 to R. Govindarjan and A. Zeitlan, and in International Application No. PCT/US01/15327 to J. B. Zeldis, et al.
In an effort to provide compounds that have greater therapeutic safety and efficacy than thalidomide, researchers have begun investigating a large number of other compounds, some of which are derivatives of thalidomide. See, e.g., Marriott, J. B., et al., Expert Opin. Biol. Ther. 1(4):1-8 (2001); G. W. Muller, et al., Journal of Medicinal Chemistry 39(17): 3238-3240 (1996); and G. W. Muller, et al., Bioorganic & Medicinal Chemistry Letters 8: 2669-2674 (1998). Examples include, but are not limited to, the substituted 2-(2,6-dioxopiperidin-3-yl) phthalimies and substituted 2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindoles described in U.S. Pat. Nos. 6,281,230 and 6,316,471, both to G. W. Muller, et al.
A group of compounds selected for their capacity to potently inhibit TNF-α production by LPS stimulated PBMC has been investigated. L. G. Corral, et al., Ann. Rheum. Dis. 58:(Suppl I) 1107-1113 (1999). These compounds, which are referred to as IMiDs™ or Immunomodulatory Drugs, show not only potent inhibition of TNF-α but also marked inhibition of LPS induced monocyte IL1β and IL12 production. LPS induced IL6 is also inhibited by IMiDs™, albeit partially. These compounds are potent stimulators of LPS induced IL10, increasing IL10 levels by 200 to 300%. Id.
In addition, nucleoside analogs have been used clinically for the treatment of viral infections and proliferative disorders for decades. Most of the nucleoside analog drugs are classified as antimetabolites. After they enter cells, nucleoside analogs are successively phosphorylated to nucleoside 5′-monophosphates, 5′-diphosphates, and 5′-triphosphates. In most cases, nucleoside triphosphates are the chemical entities that inhibit DNA or RNA synthesis, either through a competitive inhibition of polymerases or through incorporation of modified nucleotides into DNA or RNA sequences. Nucleosides may act also as their diphosphates.
5-Azacytidine (also known as azacitidine and 4-amino-1-β-D-ribofuranosyl-1,3,5-triazin-2(1H)-one; Nation Service Center designation NSC-102816; CAS Registry Number 320-67-2) has undergone NCI-sponsored trials for the treatment of MDS. See, e.g., Kornblith et al., J. Clin. Oncol. 20(10): 2441-2452 (2002); Silverman et al., J. Clin. Oncol. 20(10): 2429-2440 (2002). 5-Azacytidine may be defined as having a molecular formula of C8H12N4O5, a relative molecular weight of 244.21 and a structure of:

Azacitidine (also referred to as 5-azacytidine herein) is a nucleoside analog, more specifically a cytidine analog. 5-Azacytidine is an antagonist of its related natural nucleoside, cytidine. 5-Azacytidine, as well as decitabine, i.e., 5-aza-2′-deoxycytidine, are antagonists of decitabine's related natural nucleoside, deoxycytidine. The only structural difference between the analogs and their related natural nucleosides is the presence of nitrogen at position 5 of the cytosine ring in place of oxygen.
Other members of the class of deoxycytidine and cytidine analogs include arabinosylcytosine (Cytarabine), 2′-deoxy-2′,2′-difluorocytidine (Gemcitabine), 5-aza-2′-deoxycytidine (Decitabine), 2(1H)-pyrimidine-riboside (Zebularine), 2′,3′-dideoxy-5-fluoro-3′-thiacytidine (Emtriva), N4-pentyloxycarbonyl-5′-deoxy-5-fluorocytidine (Capecitabine), 2′-cyclocytidine, arabinofuanosyl-5-azacytidine, dihydro-5-azacytidine, N4-octadecyl-cytarabine, elaidic acid cytarabine, and cytosine 1-β-D-arabinofuranoside (ara-C).
There remains a need for more effective methods for treating MDS and its related disorders.